Overtone and combination band Raman spectra of α-copper phthalocyanine

Abstract

Compacted electrodes made by pressing α-copper phthalocyanine (αCuPc) and silver metal powder provide an effective method to measure resonance Raman scattering from αCuPc. We report first and very weak second overtone and combination bands of αCuPc. They are based on progressions in ν₃, ν₄, ν₇ and ν₁₅ modes. Three of these are A_1g vibrations of αCuPc, indicating a much stronger symmetric contribution to overtone scattering than is the case in porphyrins. Overtone profiles at 0 V vs. SCE arise mainly from scattering from 0 to 1 vibronic overtone states with evidence of higher state contributions in some cases. This is in contrast to the more varied profiles of fundamental modes and is as expected from theory. The results support the view that Raman scattering from αCuPc involves a Jahn–Teller distorted excited state and is due to A as well as B term enhancement.